In printing pictorial matter, whether by lithography, letterpress or gravure, the halftone process is used, in which the actual printed image is composed of thousands of minute dots per square inch of a single color ink of varied dot size or ink density. What the eye sees as shading in halftone prints is actually controlled variation in the size of dots relative to the unprinted areas between the dots. In black and white pictorial matter the dots are printed in black ink only. Full color reproductions, however, are printed in each of at least cyan, magenta and yellow, and, optionally, black. For each color a separate printing plate is made. In order to make the three or four printing plates, the original color picture or photograph is "separated" photographically, with the use of filters, masks, etc. into three or four half-tone negatives, each representing one of the colors, and containing, dot for dot, the amount of that color which must be printed to produce the desired total color print. The preparation of color-separation negatives is an art and requires considerable skill in handling the many variables. Often trial and error is involved, requiring correction or replacement of one or more of the negatives. Unless some reliable system is available to "proof" the negatives, the printing press must be set up and a copy printed just to secure preliminary proofs. This is time consuming and expensive. It is therefore desirable to provide accurate means for proofing the negatives without printing.
Various color proofing systems have been developed to efficiently and economically address the problem of proofing negatives. One system for proofing color separation negatives involves providing a transparent sheet for each of the colors to be printed. Each of the sheets is exposed through its respective color separation negative. Upon processing, the color in the non-image areas is removed, yielding a sheet which contains the desired color pattern in the image areas, while being colorless and transparent in the non-image areas. After each of the separate sheets is made, they are assembled together in registry on a white background to yield a color proof.
An alternate system involves producing images corresponding to each color and integrally building the images up on a single substrate. The multiplicity of carrier films is eliminated by transferring individual color images from a sheet typically comprised of(1) a carrier with release surface, (2) pigment and binder layer, (3) photohardenable or insolubilizable layer, (4) barrier layer, and (5) pressure-sensitive adhesive layer. See e.g., U.S. Pat. No. 3,671,236.
To properly anticipate the final printed image, the surface of both black-and-white and color proofs may be modified. Typically, this entails providing a varnish or gloss to the proof or providing the proof with a matte surface. A matte surface, for example, is readily provided by embossing the surface of the proof. A glossy surface may be inherent in the color image sheets or may be provided by adding an additional glossy layer or coating.
At times it may be desirable to provide gloss or matte surfaces over only a portion of the proof. For example it may be desirable to have an image of an object which has water droplets on it. One very effective way to convey the water droplets is to make those portions glossier than the rest of the image. Unfortunately, those methods previously taught for providing a glossy or a matte surface are not easily used to provide image-wise variation in gloss, i.e. a surface that is glossy in part, and matte in part. "Matte" and "gloss" are used herein as relative rather than absolute terms. Surfaces referred to as "matte" are merely visibly less glossy than are those referred to as "gloss".